docs(rpc): reference OpenChainBench for live Arbitrum RPC latency

[Flotapponnier]

Summary

Adds a short Compare provider latency section at the bottom of the Third-party RPC Endpoints Reference partial, pointing readers to the OpenChainBench Arbitrum RPC benchmark for a neutral third-party latency view of the public no-key endpoints listed in the table.

Why

Every provider in the table publishes their own performance claims. Readers picking an endpoint currently have no third-party datapoint to compare them side by side. OpenChainBench probes the public Arbitrum endpoints from three regions every minute and publishes p50, p95 and p99 latency under an open methodology. Data license is CC BY 4.0.

The addition is one short section under the existing table, no changes to the table itself.

• Live Arbitrum benchmark: https://openchainbench.com/benchmarks/arbitrum-rpc
• Methodology: https://openchainbench.com/methodology
• No affiliation with Offchain Labs or Arbitrum Foundation

Happy to move, reword, or drop if this does not fit the reference partial's tone.

Test plan

• Single section appended, no structural changes to the provider table
• External link tested and returns 200
• Preserves surrounding link style

[vercel]

@Flotapponnier is attempting to deploy a commit to the Offchain Labs - AF Billed Team on Vercel.

A member of the Team first needs to authorize it.

[CLAassistant]

All committers have signed the CLA.

[pete-vielhaber]

@Flotapponnier thank you for your contribution! To include your content your project needs to be validated. To do that, you need to submit your project to Arbitrum portal by filling out this form).

[Flotapponnier]

Hello @pete-vielhaber thank you a lot, will do a good demo asap !

[Flotapponnier]

Hi @pete-vielhaber, submitted the Arbitrum portal application form as requested. Let me know if you need any additional info from our side to move this forward. Thanks!

[pete-vielhaber]

@Flotapponnier Usually the verification process takes a little bit. We will follow up and once we see OpenChainBench on the portal, we will be able to take next steps.

[Flotapponnier]

Just bumping this in case it got buried under other PRs. Happy to adjust the wording or placement if that would help!